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Abstract:

The explosive consolidation of semiconductor powders results in
thermoelectric materials having reduced thermal conductivity without a
concurrent reduction in electrical conductivity and thereby allows the
construction of thermoelectric generators having improved conversion
efficiencies of heat energy to electrical energy.

Claims:

1. A method for making a thermoelectric material, said method comprising:
placing thermoelectric powder particles in a tube; positioning an
explosive material around the tube; and detonating the explosive material
to generate an explosive shockwave, wherein the explosive shockwave
creates pressure equal to or greater than 3 GPa, and the explosive
shockwave consolidates the thermoelectric powder particles into a solid
body.

2. The method of claim 1, wherein the method occurs adiabatically.

3. The method of claim 1, wherein the thermoelectric powder particles
comprise a nanopowder having dimensions under 20 nm.

4. The method of claim 1, further comprising adding a quantity of an
electrode material to the tube.

5. The method of claim 4, wherein the thermoelectric powder particles and
the quantity of the electrode material are arranged in a plurality of
alternating layers.

6. The method of claim 4, wherein the electrode material is a powder.

7. The method of claim 4, further comprising cutting through each of the
plurality of electrode material layers after the detonating step.

8. The method of claim 1, wherein the thermoelectric powder particles
comprise an element selected from the group consisting of bismuth,
tellurium, antimony, selenium and combinations thereof.

9. The method of claim 1, wherein the thermoelectric powder particles
comprise a plurality of different thermoelectric substances, and each of
the plurality of different thermoelectric substances is arranged as a
separate layer.

10. A method for making a thermoelectric material, said method
comprising: placing thermoelectric powder particles in a tube;
positioning an explosive material around the tube; and detonating the
explosive material to generate an explosive shockwave, wherein the method
occurs adiabatically, and the explosive shockwave consolidates the
thermoelectric powder particles into a solid body.

11. The method of claim 10, wherein the explosive shockwave creates
pressure equal to or greater than 3 GPa.

12. The method of claim 10, wherein the thermoelectric powder particles
comprise a nanopowder having dimensions under 20 nm.

13. The method of claim 10, further comprising adding a quantity of an
electrode material to the tube.

14. The method of claim 13, wherein the thermoelectric powder particles
and the quantity of the electrode material are arranged in a plurality of
alternating layers.

15. The method of claim 13, wherein the electrode material is a powder.

16. The method of claim 13, further comprising cutting through each of
the plurality of electrode material layers after the detonating step.

17. The method of claim 10, wherein the thermoelectric powder particles
comprise an element selected from the group consisting of bismuth,
tellurium, antimony, selenium and combinations thereof.

18. The method of claim 10, wherein the thermoelectric powder particles
comprise a plurality of different thermoelectric substances, and each of
the plurality of different thermoelectric substances is arranged as a
separate layer.

19. A method for making a thermoelectric material, said method
comprising: placing thermoelectric powder particles in a tube; capping a
first and second end of the tube; placing the tube into an outer
container; positioning an explosive material in the outer container
around the tube; and detonating the explosive material to generate an
explosive shockwave, wherein the explosive shockwave creates pressure
equal to or greater than 3 GPa, the method occurs adiabatically, and the
explosive shockwave consolidates the thermoelectric powder particles into
a solid body.

20. The method of claim 19, further comprising adding a quantity of an
electrode material to the tube, wherein the electrode material is a
powder.

21. The method of claim 20, wherein the thermoelectric powder particles
and the quantity of the electrode material are arranged in a plurality of
alternating layers.

22. The method of claim 21, further comprising cutting through each of
the plurality of electrode material layers after the detonating step.

23. The method of claim 19, wherein the thermoelectric powder particles
comprise a plurality of different thermoelectric substances, and each of
the plurality of different thermoelectric substances is arranged as a
separate layer.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent application Ser.
No. 12/220,156 filed Jul. 22, 2008, which claims priority to U.S.
provisional patent application 61/007,319 filed Dec. 12, 2007, each of
which are incorporated by reference herein in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

FIELD OF THE INVENTION

[0003] This invention relates to the shockwave fabrication of
thermoelectric materials as a means for deriving an enhanced performance.
The consolidation of micro and nano-scale powders through explosive means
results in a material that impedes thermal energy transmission through
lattice vibrations (phonons) without negatively impacting electrical
conductivity, the result being an improvement in thermoelectric
conversion efficiency.

BACKGROUND

[0004] Energy is a quantity that has many forms, with electrical energy
having the attractive property that it may be easily transmitted through
conductors and thereby transferred to remote locations without the
requirement for mechanical transport. Electrical energy may be used to
generate mechanical motion through motors, and can power sensors,
electronics and heaters. A less versatile manifestation of energy is
thermal energy, or heat. Thermal energy may be produced as a byproduct of
exothermic chemical reactions, such as the combustion of hydrocarbons in
an internal combustion engine. Thermal energy may be derived from hot
springs and vents in the earth's crust (geothermal energy) and as a
byproduct of radioactive decay. Thermal energy may also be collected as
the absorption of solar radiation.

[0005] Thermoelectric generation is the conversion of thermal energy into
electrical energy without the intermediary of rotating machinery. Status
quo thermoelectric generators are constructed from thermoelectric
elements that are generally formed of doped semiconductors. Even the best
of these suffers from inefficiencies that render them unusable for all
but a limited class of applications. Transport mechanisms in solid state
materials are complicated, interrelated and require detailed quantum
mechanical calculations to describe. However, for the purposes of the
following discussion we will focus on the effect of lattice vibrations on
the thermoelectric qualities of a material. These mechanical excitations
of the atoms in a solid are called phonons and they have two main
undesirable effects in a thermoelectric material. The first effect is
that they transmit heat directly through a material, effectively acting
as a leak for thermal energy. The second effect is to scatter electrons
as they travel through the material, effectively causing electrical
resistance and joule heating. The present invention describes a
fabrication method using shockwave powder consolidation that reduces
lattice vibrations, leading to an improved thermoelectric performance.

[0006] Thermoelectric generation takes place when a temperature difference
is applied to a conductor or semiconductor and causes mobile charge
carriers, either electrons or holes, to migrate from hot to cold. The
resulting separation of charge creates an electric field potential known
as the Seebeck voltage, that is given by ΔV=αΔT, where
α is a temperature dependent material property known as the Seebeck
coefficient and, by convention, ΔT represents the temperature of
the cold side with respect to the hot side. The Seebeck coefficient for a
material may be positive or negative depending upon the type of majority
charge carrier.

[0007] For thermoelectric devices operating around room temperature
(300K), it is common to use doped alloys of tellurium, as the active
elements for converting thermal energy to electrical energy. Doped
bismuth-telluride (BiTe) alloys are the most popular for a temperature
range of about 220K to 420K and have the attractive properties of a
relatively high Seebeck coefficient α, a relatively high electrical
conductivity, σ, and a relatively low thermal conductivity
λ=λe+λl where λe=LTσ is the
portion of thermal conductivity due to electrons with T being temperature
and L being the Lorentz constant, and λl being the portion of
thermal conductivity due to lattice vibrations (phonons). These bulk
material properties are often lumped into a single figure of merit Z,
where

Z = σα 2 λ e + λ 1 . ( 1 )
##EQU00001##

In general, the higher the value of Z, the better the thermoelectric
conversion efficiency. The parameters α, σ, and λl
are temperature dependent.

[0008] Thermoelectric materials have traditionally been manufactured
through a bulk process. Bulk fabrication techniques include melting and
powder 10 compaction. However, for any given material, the constituents
of the figure-of-merit, Z, namely, α, λ, and S, are tightly
intercoupled, so that changes that are made in chemistry or crystalline
structure that influence one of these parameters in a positive way are
generally offset by a negative influence on another parameter. By
contrast, device fabrication on the nanoscale (with feature sizes under
20 nanometers) alters the relationship between the various constituents
of Z, enabling another variable for optimization.

[0009] Z is not the only important metric for analyzing a TE device and
is, at best, an average quantity. Often, thermoelectric materials are
discussed in terms of the dimensionless product ZT in order to have a
common reference point when discussing materials that have been optimized
over different temperature ranges.

[0010] There must be an electron flow in a thermoelectric generator since
the object is to supply electrical current. As such, electronic heat
transfer is unavoidable and most of the focus in reducing thermal
conduction has been directed at reductions in lattice (phonon) transport.
Given a specific chemical make-up, thermoelectric materials may be
fabricated as a single crystal, in polycrystalline form or as an
amorphous (non crystalline) form. In electrically conductive substances,
crystals are generally good material structures for both electron and
heat transmission. Their regular structure promotes long mean free paths
(mfp's) which are the mean distances that an electron or phonon travels
between collisions. In contrast with a crystal, a glass exhibits no
ordering between molecules and is said to have an amorphous structure.
Accordingly, it has been proposed that a desirable property for a
thermoelectric will be that it resembles a phonon glass and an electron
crystal [G. A. Slack in CRC Handbook of Thermoelectrics, CRC Press, 1995,
p. 411]. Much research has been invested into methods that scatter
phonons (thus reducing their mean free path) more effectively than
electrons.

[0011] One approach to building low thermal conductivity materials is to
use powder sintering. The constituent materials are ground into a powder,
then combined by compaction and sintering (heating). These constituent
materials may be provided as individual powdered elements and then mixed
and compacted and sintered. Alternately, the constituents may be prepared
as a melt and then ground into a powder that is compacted and sintered.
The approach of powder compaction and sintering is said to introduce
disorder, lattice defects and grain boundaries which will inhibit phonon
transport without excessively compromising electron transport.

[0012] U.S. Pat. No. 3,524,771 to Green describes a method for preparing
thermoelectric materials consisting of small particles that are sintered
to form a solid element. This approach is said to reduce thermal
conductivity but at the expense of reduced electrical conductivity. U.S.
Pat. No. 5,411,599 to Horn et al, describes a technique for fabricating
thermoelectric materials with low thermal conductivity whereby a
nanoporous structure is achieved by chemically preparing particles of a
bismuth telluride based alloy and then compacting these particles. The
resulting device is said to have nanoinclusions which lead to a reduced
phonon conductivity. U.S. Pat. No. 6,319,744 B1 to Hoon et al., describes
a technique for manufacturing thermoelectric semiconductor material by
laminating strips of thin powders and then compressing and sintering to
form a composite solid. U.S. Pat. No. 6,596,226 B1 to Simard describes a
compaction method for alloying constituent powders in order to devise
thermoelectric devices. The procedure consists of mechanically alloying
the constituent elements in a powder form, compacting the resulting
powder, heat treating the alloy and then extruding the resultant device.
U.S. Pat. No. 7,365,265 B1 to Heremans et al, describes a technique to
build thermoelectric elements using a nanogranular material which is
compressed and sintered.

[0013] It is important to note that all previously proposed techniques for
constructing thermoelements from powders have utilized compaction
techniques as opposed to explosive consolidation. Although samples
prepared using the two techniques can have identical particle densities,
a consolidated sample will have maximum particle to particle bonding [K.
P. Staudharnmer and L. E. Murr, "Principles and applications of shock
wave compaction and consolidation of powdered materials", in Shock Waves
for Industrial Applications, L. E. Murr, Editor, Noyes Publications,
1988, p. 2381. Also, when powder compaction is combined with sintering,
the sintering occurs at comparably large temperatures, effectively
annealing the particles. Explosive consolidation is, in effect, a
relatively low temperature procedure that is not conducive to crystal
formation.

[0014] U.S. Pat. Nos. 4,717,627 and 4,907,731 to Nellis et al describe a
shockwave consolidation approach to the fabrication of fine grain
materials having desirable superconducting or magnetic properties. U.S.
Pat. No. 5,129,801 to Rabin et al describes an explosive consolidation
for powders which uses an explosively propelled piston to compress a
pelletized powder with particular applications to titanium carbide and
alumina. These inventions propose shockwave consolidation as a means to
fabricate homogeneous monoliths and do not suggest the thermoelectric
advantages that will accrue from a reduction in thermal transport.

SUMMARY

[0015] In order to reduce the mean free path of phonons it is desirable
for the semiconductor thermoelectric to have small crystalline structure,
that is, many grain boundaries that would serve to block phonon
transmission while preserving the ability for electron transmission. In
this sense, we desire a "pseudo-glassy" material. One way to accomplish
this is through shock wave consolidation of a powder mixture. As
contrasted with powder compaction and sintering techniques, consolidation
allows complete particle to particle bonding, thus producing a monolith
of homogeneous properties. Shock consolidation requires the very rapid
collapse of the gaps between the powder particles as well as the rapid
deposition of energy at the particle surfaces. These processes must be
close to adiabatic. The ultra rapid deformation and energy deposition is
accomplished in time durations of microseconds by the passage of an
explosively produced shock wave. The amplitude of the shock wave has to
be sufficient to bond the powders but should not be so high as to produce
extensive melting and cracking on subsequent reflections. The correct
choice of explosive material and experimental geometry can ensure an
acceptable result.

[0016] Of particular interest is the ability of shockwave fabrication to
consolidate so-called nanopowders. Nanopowders are powders for which a
significant proportion of the constituent particles have dimensions under
20 nanometers.

[0017] Prior art approaches to the preparation of thermoelectric materials
using melts will exhibit a polycrystalline structure that does nothing to
inhibit phonon transport. Prior art approaches that use powder compaction
will have limited interparticle bonding. Accordingly, the present
invention has the following objects and advantages for the manufacture of
thermoelectric materials: [0018] a. It maximizes interparticle bonding
and thereby enhances electrical conductivity; [0019] b. Thermoelectric
materials are produced adiabatically, discouraging the formation of
crystalline structures; [0020] c. It results in a reduced lattice thermal
conductivity; and [0021] d. It yields an improved performance
thermoelectric material.

[0022] None of the prior art embodiments, either alone or in combination,
anticipates the present invention. Other objects, advantages and novel
features, and further scope of applicability of the present invention
will become apparent to those skilled in the art upon examination of the
following, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and attained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.

[0049] FIG. 1 depicts the side view of a typical thermoelectric generator.
The generator is constructed by sandwiching specially chosen n and p type
conductor or semiconductor material (the thermoelements) between
electrical conductors 10. Although thermoelements may be built using
conductors such as bismuth and antimony, higher efficiency
thermoelectrics are built using heavily doped semiconductors. The
electrical conductors 10 will be chosen to be good conductors of both
electricity and heat. When fabricated from a semiconductor material, the
n type thermoelement 12 is formed by the introduction of a pentavalent
chemical compound so that electrons are the majority carrier. When
fabricated from a semiconductor material, the p type thermoelement 14 is
formed by the introduction of a trivalent chemical compound so that the
majority carriers are holes. When the thermoelectric device is placed
between a heat source 16 and a heat sink 18, there is a flow of heat
energy from the source 16 to the sink 18. In FIG. 1, the thermoelements
are connected in electrical series and thermal parallel. As heat flows
from the heat source 16 to the heat sink 18, the thermal current excites
an electrical current, I, which flows through a conductor 19 to an
attached electrical load 20. A key figure-of-merit for thermoelectric
materials is the so-called "Z" which is given by

Z = σα 2 λ ##EQU00002##

where σ is electrical conductivity, α is the Seebeck
coefficient and governs the amount of voltage that is produced for a
given temperature gradient (V=αΔT) and λ is the
coefficient of thermal conductivity. High values of Z are desirable and
this provides a roadmap for improving thermoelectric performance, namely,
increase σ and α and decrease λ. At temperatures in the
range of 250K to 400K, alloys of bismuth-telluride exhibit the highest
values of Z. N and p type thermoelectric elements are then produced by
heavy doping with selenium and antimony respectively. Published desirable
stochiometries for n and p type thermoelectrics are given in
Thermoelectrics Handbook, Macro to Nano, D. M. Rowe, editor, CRC Press,
Boca Raton, Fla., 2006, p. 27-9 as (Bi2Te3).sub.95 (Bi2Se3)5
for n type and (Bi2Te3).sub.75(Sb2Te3)25 for p
type.

[0050]FIG. 2 depicts a Peltier heat pump. This is a thermoelectric device
that can be of identical construction to the thermoelectric generator of
FIG. 1, with the difference being that instead of a load, there is a
voltage source 22 that causes current flow. The coupling between heat
currents and electrical currents in a thermoelectric device results in
the movement of heat from a first side 24 of the device to a second side
26 with the result that the first side 24 becomes cooler than the ambient
temperature and the second side 26 becomes warmer than the ambient
temperature. Just as for a thermoelectric generator, the n type
thermoelements 28 and p type thermoelements 30 are optimized though an
optimization of the figure-of-merit Z.

[0051]FIG. 3 depicts an apparatus for the explosive consolidation of
thermoelectric powders. An outer container 46 mounted onto a base 44
supplies mechanical support. A metal tube 38 holds the thermoelectric
powder 40 which is to be consolidated. The thermoelectric powder 40 may
be prepared by crushing and milling a melt grown ingot that has the
desired chemical composition, or the thermoelectric powder 40 may be
prepared through a sol synthesis approach to chemically produce
nanopowders of the correct composition, or the thermoelectric powder may
be simply a mixture of elemental powders in the correct stochiometric
ratio, in which case the shockwave consolidation will serve to
mechanically alloy the powders into a uniform monolith having the desired
make-up. The tube 38 is capped at either end with endcaps 36. The outer
container 46 is then filled around the tube 38 and to the top of the
outer container 46 with an explosive material 42 such as ammonium nitrate
fuel oil mixture (ANFO). The explosion is initiated by a fuse 32 which
sets off a detonator 34 that in turn detonates the explosive material 42.

[0052] Thermoelectric materials may be shock consolidated through the
application of an explosive pressure pulse to a cylindrical container 38
of powder 40. The thermoelectric powder 40 to be consolidated is first
placed within the tube 38 and green compacted. Endplugs 36 are applied to
seal the tube and to maintain the powder 40 in a compacted state. The
outer container 46 is not critical for strength but merely serves to
contain the explosive materials 42 prior to detonation. Upon detonation,
the pressure pulse converges toward the central axis of the cylinder.
Shock waves cause pressures on the order of 3 to 7 GPa, causing
consolidation of the powders into a solid without voids.

[0053] The result from implementing shockwave consolidation on a sample
using the apparatus depicted in FIG. 3 is a cylindrical monolith of the
consolidated material within the container 38. The container 38 is
removed by machining off the outer layer. Thermoelectric elements may be
prepared by simply cutting off slices like cutting a cucumber. These
slices are then attached to electrodes to fabricate the arms of a
thermoelectric generator.

[0054]FIG. 4 depicts an alternative means for loading a tube 40 prior to
explosive consolidation. Endcaps 36 are still used on the top and bottom
of the tube 40. The tube 40 is loaded with alternating layers of
electrode 50 and green compacted thermoelectric powder 48. Detonation
causes consolidation between the thermoelectric and the electrodes 50.
The result is a roll that can be cut through the centers of the
electrodes 50 to yield thermoelectric elements that are already attached
on either end to conductive electrodes. It is then easy to connect
multiple elements in series or in parallel in order to obtain a required
performance. The advantage to this approach is that the adherence between
semiconductor thermoelectric material and the electrodes is performed as
part of the process and is done without the requirement for secondary
processing such as soldering. Another advantage is that this technique
can be used as a means for producing so-called functionally graded
thermoelectric elements whereby different zones within a given element
are optimized for different operating temperatures. Simply use multiple
layers of powder, each layer corresponding to a desired Z. The shock wave
causes fusing of the various layers in a continuum. The resulting
thermoelements would have a preferred orientation when used between
thermal reservoirs.

[0055] One potential problem in achieving a bonding between the
thermoelectric constituent powders and the electrodes is achieving a
matched mechanical impedance. With a poor match (as can occur between a
powder and a solid), the shock reflections may prevent a good electrical
and mechanical connection between electrode and the active thermoelectric
material. One alternative is to use powder for both the thermoelectric
material and for the electrode. For example, by alternating layers of
thermoelectric and nickel powder in a tube container, the explosive
consolidation could be relied upon to create a single cylindrical
monolith which could be sliced into multiple coinlike thermoelements for
incorporation into a thermoelectric generator.

[0056] Although the invention has been described in detail with particular
references to these preferred embodiments, other embodiments can achieve
the same results. Variations and modifications of the present invention
will be obvious to those skilled in the art and it is intended to cover
in the appended claims all such modifications and equivalents. The entire
disclosure of all references, applications, patents, and publications
cited above are hereby incorporated by reference.